The Environmental Protection Agency (“EPA”) is proposing regulations relating to the mercury content in emissions from coal-fired power plants. In December 2000, EPA announced that it would propose regulations by 2003 and issue final rules by 2004. The goal of the U.S. Department of Energy (“DOE”) is to cut mercury emissions by 50 to 70 percent by 2005 and by 90 percent by 2010. The level of mercury in coal and the resultant flue gas from the burning of coal, such as in coal-fired plants, is so low that economical accurate analysis at this level was not possible until the last decade. However, the annual global release of mercury into the earth's atmosphere from all sources is estimated to be 5,000 tons per year of which 4,000 tons per year is estimated to occur as a result of human activities. The United States emissions of mercury from all sources are estimated to be 158 tons per year. Coal-fired power generation is the largest single contributor to U.S. mercury emissions at an estimated rate of 50 tons per year. The level of mercury in United States coal ranges from about 20 to about 300 parts per billion by weight (ppbw).
Mercury is the most volatile of all metals, having a normal boiling point of 642° F., well below the melting point of most metals. Mercury is conventionally associated with the inorganic fractions of the combustible material such as coal and is normally in the form of mercury oxide, HgO, mercury sulfide, HgS, mercury chloride, HgCl2, and the like. In this form, coal washing typically removes as much as 30 percent of the mercury found in a normal run of mine coal thus leaving on the order of 70% of the mercury content in the coal. Coal combustion usually results in a flame temperature of at least 2,000° F., which dissociates all known mercury compounds to vaporized elemental mercury. The mercury is then carried in the flue gas and, unless otherwise treated, is released into the atmosphere. As the combustion gases cool, mercury may react with other components of the combustion gas to form mercury oxide, HgO, mercury sulfide, HgS, mercury chloride, HgCl2, and the like. These compounds can be removed from flue gas more easily than elemental mercury because they desublime, that is they form a solid directly from vapor and can thus be removed with other particulate material. Assuming that during combustion all mercury found in coal does become elemental mercury in the flue gas, the typical vapor concentration of mercury is estimated to be 2.2 parts per billion (ppb) by volume. Although mercury is most targeted for removal from emission products, other trace metals, such as selenium and cadmium can also be present in the emissions produced by the combustion of carbonaceous materials and their emission can also be harmful to the environment.
Various methods have been proposed in the prior art for the abatement of mercury in flue gas emissions. For example, U.S. Pat. No. 5,435,980 issued Jul. 25, 1995 to Felsvang, et al. relates to a method for mercury abatement from coal-fired power plants flue gases using a spray drying absorption system employing the enhancement of the chloride content of the flue gas to form mercury chloride which can then be separated from the flue gas.
U.S. Pat. No. 5,607,496, issued Mar. 4, 1997, to Richard J. Brooks discloses a process in which the mercury of a hot combustion stream gas is oxidized to mercury oxide and then subsequently absorbed on particles that can be regenerated and reused by heating to decompose and drive off the mercury compounds. In one embodiment, the oxidation of the elemental mercury is catalytically promoted and the mercury compounds are removed from the gas stream by scrubbing.
U.S. Pat. No. 6,136,281, issued Oct. 24, 2000 to Meischen, et al. discloses a similar method for control of mercury emissions by the oxidation of elemental mercury in a flue gas stream prior to standard emissions control equipment. In its oxidized form, mercury can be more efficiently removed from flue gas streams by wet processes or by absorption or by wet processes. In one embodiment oxidation of the mercury takes place by the use of a porous bed of gold-coated material saturated with elemental mercury to the point that the gold in the presence of hydrochloric acid in the exhaust stream catalyzes the oxidation of elemental mercury.
U.S. Pat. No. 6,156,281, issued Dec. 5, 2000, to Akers, et al. relates to a process for removal of mercury and other trace elements from coal containing pyrite by forming a slurry of finely divided coal in a liquid solvent capable of forming ions or radicals having a tendency to react with constituents of pyrite or to attack the bond between pyrite and coal and/or to react with mercury to form mercury vapors. The slurry is heated in a closed container to a temperature of at least about 50° C. to produce vapors of the solvent and withdrawing vapors including solvent and mercury-containing vapors from the closed container then separating mercury from the vapors withdrawn, such as by treatment in a sulfuric acid bath to form mercury sulfide. This process is applied to the coal prior to its combustion.
U.S. Pat. No. 6,103,205 Wojtowicz, et al. relates to a process involving the regenerative absorption of mercury on activated carbons derived from scrap tires that inherently contain appreciable and desirable amounts of sulfur. This process also results in the concurrent control of SO2 and NOx.
In European patent no. 0 253 563 a method for removal of mercury and other noxious compounds from incinerator flue gas is disclosed in which an aqueous liquid containing a basic absorbent is atomized into the flue gas to absorb acidic components from the flue gas and simultaneously to evaporate the water in said aqueous liquid, in which process powdery activated carbon is injected into the flue gas and separated again from said gas together with particulate material formed as a result of chemical reactions and drying of the atomized basic absorbent.
U.S. Pat. No. 3,662,523, Revoir et al., issued May 16, 1972 discloses that mercury removal from a gas is enhanced by passing the gas through a bed of carbon impregnated with halogen or inter-halogen compounds. The '523 process was primarily directed at small volumes of gas and/or high concentrations of mercury, with end use mentioned in the patent as “ . . . on respirators to protect personnel against inhalation of mercury vapor or on large industrial processing equipment containing mercury to prevent contamination of the atmosphere with mercury vapor.
The “state of the art” seems to be directed to post-combustion treatment of the mercury in the flue gas from coal (or flue gas from other fuels which contain Hg, such as heavy oil and waste streams). Use of scrubbers or injecting chemicals such as halogens to change the chemical form of at least some of the mercury and improve capture efficiency in the scrubber or conventional adsorbent. Some approaches used an adsorbent with additives such as a halogen, found to improve Hg recovery. Some researchers even turned to a “gold plated” solution, using gold to form an amalgam with mercury and improve recovery.
These conventional approaches were expensive, because of the large volumes of gas to be treated and the low concentration of mercury and mercury compounds in the gas. Gas treatment was also complicated to some extent because the mercury would typically be in multiple oxidation states, ranging from elemental metal to some or all of it being in various oxidation states. Some work was done on injecting light hydrocarbon gas into flue gas to get more of the mercury in elemental form.
I wanted to have a more efficient way to remove mercury from, e.g., flue gas from burning of coal. I realized that conventional approaches to burning, e.g., oxidize the coal and release the at least partially oxidized mercury in an even more diluted form in the flue gas, just made mercury removal more difficult. The conventional approaches could be compared to using various scrubbers and/or adsorbents to remove SOx from flue gas. I realized it would be better to remove the sulfur prior to combustion, rather than after, because it is cheaper and more efficient. To have low sulfur emissions from burning gasoline, or fuel oil, it is much cheaper to remove the sulfur from the feed prior to combustion, rather than after.
Removing mercury from a feed stream is easier said than done. Conventional hydrotreating removes sulfur and may remove some mercury from a heavy oil feed, but this approach is not even an option with coal. I realized that another approach—thermal rather than catalytic—could be used to efficiently remove mercury from feedstocks and have the mercury in a relatively concentrated form, free of solids, and in the form of metal rather than an oxide. A thermal process which can be used to process coal, or any other feed with carbon, hydrogen and volatile metallic contaminant, has been developed by EnviRes, LLC, the HyMelt® process. HyMelt is ideal for severe thermal processing of coal, or other carbon, hydrogen and mercury containing feeds in that it separates high temperature thermal processing from oxidation. Thermal processing, in a relatively reducing atmosphere, provided the key step, which allowed mercury to be, in effect, “squeezed out” of a combustible chargestock, and permit its capture in a gas stream free of solids and condensable hydrocarbons. In addition to “squeezing out” the mercury, the high temperatures used thermally decompose the decomposable parts of the feed. In the case of a heavy hydrocarbon oil, used for the relative simplicity of its chemical composition, thermal decomposition produces large amounts of relatively pure hydrogen gas and solid carbon. The solid carbon dissolves in the molten metal bath while the hydrogen, with the vast majority of the mercury present in the feed heavy oil, is removed and recovered as a vapor phase product. Thermal decomposition creates reducing conditions which ensure that the mercury will be in the form of metal. Thermal decomposition or processing creates significant volumes of relatively clean gas, which carry the mercury away from the process. The large volumes of clean gas allow some heat recovery from this gas stream by indirect heat exchange without condensing the mercury.
Thermal processing, in the case of coal, reduces the volume of gas which has to be treated by at least 2 and typically by a factor of 5, or 10, or even 15 or more, as compared to the volume of flue gas produced when a like amount of coal is simply burned. Increasing the pressure of the thermal decomposition vapor stream is also possible, either by running the thermal processing step at superatmospheric pressure or by compressing the gas, usually after heat recovery, and charging the compressed gas to mercury recovery.